The purpose of this project is to develop practical and efficient procedures for isolating the placental (trophectoderm) and fetal (inner cell mass) cellular precursors from the developing blastocyst and then successfully reconstruct the blastocyst. This process, known as blastocyst reconstitution, likely is important in attempting to overcome fetal rejections associated with interspecies embryo transfer. By injecting the inner cell mass of one species (or genotype) into the trophectoderm vesicle of the recipient genotype, it may be possible to eliminate fetal loss associated with immunological rejection and, thus, increase the reproductive potential of a valuable genotype (e.g., rare animal model or endangered species). The proposed experiments will answer specific questions, while developing effective procedures for isolating zona pellucida-enclosed trophectoderm vesicles using physical and chemical methods and inner cell masses using heterologous antibody sources.